1. Field of the Invention
The present invention is directed to a continuous process for the production of stable, aqueous dispersions of polyurethane-ureas using low shear mixers.
2. Description of the Prior Art
The production of aqueous polyurethane-urea dispersions is known as shown by U.S. Pat. Nos. 3,479,310; 4,066,591; 4,092,286; 4,108,814; 4,237,264; and 4,238,378 which disclose linear polyurethane-ureas and U.S. Pat. Nos. 3,870,684; 4,203,883; and 4,408,008 which disclose cross-linked polyurethane-ureas. In general, the dispersions prepared in accordance with these patents are prepared by a batch process wherein an isocyanate-terminated prepolymer is formed in one reaction vessel, is dispersed in water in the same or a different reaction vessel and is finally chain-extended to form the aqueous polyurethane-urea dispersion.
One of the difficulties with the batch process is that the large reaction vessels required for commercial production require powerful stirring means to both disperse the isocyanate-terminated prepolymer and to chain extend the dispersed prepolymer. In addition, the more rapid rate of the reaction between aromatic isocyanate groups and water makes it extremely difficult to prepare aqueous polyurethane-urea dispersions from prepolymers containing aromatically-bound, terminal isocyanate groups. Further, the properties of the dispersions vary from batch to batch because of the complexity of the chemical reactions taking place, sometimes even resulting in products which are not usable. Finally, it is often difficult to scale up a particular dispersion from a laboratory scale to a commercial scale and still maintain the properties and appearance of the product as produced on the smaller scale.
Continuous processes for the production of aqueous polyurethane-urea dispersions are known as shown by British Pat. Nos. 1,414,930; 1,432,112; and 1,428,907 and German Offenlegungsschrift No. 2,347,299. However, these processes are dependent upon the use of high shear mixers requiring large amounts of energy for their operation. In addition, the shearing forces necessary for dispersing and chain-extending the prepolymer have a detrimental effect on the properties of the dispersion.
Accordingly, it is an object of the present invention to provide a continuous process for the preparation of aqueous polyurethane-urea dispersions having properties and an appearance which are consistently reproducible. It is an additional object of the present invention to provide a continuous process which does not have the large power requirements of the known batch processes or high shear processes. It is a further object of the present invention to provide a continuous process for production of aqueous polyurethane-urea dispersions which is easy to scale up from laboratory scale to pilot plant scale and, ultimately, to commercial size production. It is a final object to provide a simple process wherein aromatic diisocyanates may be used without substantial reaction of the aromatic isocyanate groups with water.
These objects may be achieved according to the present invention by using special low shear mixers as hereinafter described.